1. Field of the Invention
This invention relates generally to the field of modems suitable for data communications at varying data transmission rates. More particularly, this invention relates to a data modem which adaptively adjusts its transmission rate in accordance with line quality to optimize data throughput over changing transmission lines.
2. Background of the Invention
It is known in the art that data communication over telephone transmission lines may be accomplished at data rates which are related to the quality of transmission line being utilized. That is, higher quality conditioned transmission lines are generally capable of transmission rates greater than those of, for example, simple voice grade transmission lines. Line quality varies significantly even among transmission lines which supposedly have similar conditioning. In addition, the quality of a particular leased circuit may vary from day to day. It is therefore possible to achieve higher rates of transmission over some lines than others due to variables such as line length, weather, time of day and age of the lines. As such, it is advantageous to provide a mechanism for measuring and monitoring line quality and having data modems adapt to the quality of lines so that data throughput is maximized for a given set of lines and line conditions.
Modem manufacturers have recognized the above problems and have proposed various circuits which attempt to maximize data throughput. For example, U.S. Pat. No. 4,4387,511 to Baran discloses a high speed modem suitable for dialup telephone lines in which the telephone pass-band is subdivided into 64 sub bands each with its own carrier. Each carrier is individually amplitude and phase modulated in order to transmit at a relatively low data rate on each carrier. The net result is a relatively high data rate. Baran's modem determines during a training period which portions of the telephone line spectrum may be most effectively utilized to carry data. Those carriers which are most impaired, are dropped in order to reduce the effective data rate while maximizing data throughput for a given set of channel impairments. Unfortunately, the Baran modem is not currently compatible with existing network control and diagnostic systems such as the Racal-Milgo.RTM. CMS series of network control and diagnostic equipment. Also, Baran's modem is only able to measure line conditions during its initial training period. Any line improvements occurring after that training period will not be recognized or accounted for, thus possibly reducing effective utilization of Bandwidth.
Other modem manufacturers have provided mdems which are capable of speed changes to adapt to varying line conditions also. However, such designs have generally failed to recognize and deal with the fact that the quality of a four wire line can vary such that line quality is high one direction of transmission (one pair of wires) and low in the other direction. Furthermore, this condition can change so that the situation is reversed. These systems are not designed to function optimally with the above mentioned advanced diagnostic and network control systems. In addition, some such systems may be actually detrimental to data throughput by continually interrupting user data in an attempt to convert to a higher data speed on a marginal or asymmetrical set of transmission lines.
Another problem with such modems is a less than optimal speed change protocol which can result in two modems temporarily winding up at different speeds which cause disruption of user data until a reinitialization occurs. This results primarily when one modem which is assigned absolute control over speed increases at any given time blindly assumes that it upspeed command correctly reaches its counterpart. An upspeed command which is not properly received due to, for example, phase hits on the line can cause such situations. It is also possible that power failures, brown outs and the like can cause neither modem to retain control over increases in speed so that a pair of modems is locked at a speed which is lower than the optimum transmission rate.
The Codex.RTM. model 2660 modem uses an interruptive inband signaling as part of its speed change protocol. No secondary channel signaling is used. While this modem has a mechanism for switching its master/slave relationship similar to the present invention, the protocol for speed changes is less than optimal in that it allows one modem to demand upspeed without consulting the other. This can result in two modems operating at different speeds if the command to change speeds is damaged in transmission. It also has less immunity to confusion over master/slave identity or transmission speed in the event of brownout or damage to control signals in transmission.
While the above problems are not exhaustive of those which are encountered in modems having adaptive speed features, they serve to illustrate a substantial need for improvement in this area. The present invention alleviates many of these problems as well as providing a protocol with minimal user data interruption as well as minimizing secondary channel interruption while providing a high degree of reliability in speed change signaling.